The use of chemicals as a potential threat in modern warfare has generated a need to detect the presence of these materials as quickly as possible so that military personnel can take necessary precautionary measures. It is known that all threat and most pollutant chemical vapors have absorption features in the 8-12 micron region. Since prior art standard Thermal Imagers (TIs) view this whole wavelength region at once, a vapor signature would represent only a small amount of energy and be difficult to detect. In addition, all vapors that absorb on the 8-12 micron wavelength band would yield the same type of image data making it very difficult to differentiate one vapor from another. In order to discriminate chemical species one must divide the 8-12 micron band into a large number of small regions so that these may then be analyzed relative to one another. Presently the Thermal Imagers that exist in the armed forces are used for tactical target acquisition, tracking and fire control and are known as Forward Looking Infrared (FLIRs). It would be a great financial and logistical advantage if these prior art FLIRs could be used as an adjunct chemical vapor detection sensor.
The problem with prior art Thermal Imagers is division of the 8-12 micron band into smaller parts can only be done with filters or a dispersion optic. Both of these approaches are not satisfactory because they both have transmission losses. In the case of filters, the pass band may be as narrow as 1/2 micron and still yield 80% transmission. Filters much narrower than 1/2 micron quickly degrade in peak transmission. Under these conditions the filter would decrease the total energy incident on the array detector, thus lowering overall sensitivity.
There are two problems with using standard band pass filters. Firstly, in order to divide the 8-12 micron band fully into 1/2 micron wide segments would require 8 individual filters These individual filters need to be mechanically rotated into the field of view sequentially to obtain spectral data, which is difficult to do, or there would have to be 8 to 10 single band filtered detectors and some method of scanning the field of view over each. The second problem with using standard band pass filters is that the prior art detector now views a "hot" filter element which is opaque over much of the sensitivity range of the detector. This is a problem particularly if the scene background is colder than the filter, it would result in considerable loss of sensitivity. The problem specific to tactical military FLIRs is the requirement for excellent spatial resolution for target acquisition and recognition. It is very important that the image quality and operational availability of the tactical sensor not be comprised in any way by the addition of further missions or hardware.